In their study published Monday in the Proceedings of the National Academy of Sciences, the researchers used functional magnetic resonance imaging to monitor the brain activity of 18 straight men as a female research participant gently stroked their legs.

The men could not see the woman. Although the same woman caressed their legs, the first time the men were told an attractive woman was caressing them, and the second time they were told it was a man. Before each part of the experiment, they were shown a video of how to visualize the person caressing their leg, although, unknown to the participants, the image was not true to the actual person.

The researchers found that although the same person was giving the caresses, a part of the midsection of the brain called the primary somatosensory cortex became more active when the men believed an attractive woman was touching them as opposed to a man.

"Intuitively, we all believe that when we are touched by someone, we first objectively perceive the physical properties of the touch - its speed, its gentleness, the roughness of the skin," said Valerie Gazzola, a co-author of the study. "Only thereafter, in a separate, second step based on who touched us, do we believe we value this touch more or less."

But the findings suggest that the primary somatosensory cortex is less objective than previously believed, and that the two parts to processing touch - one of understanding the physical component, and the other of assigning emotion to it - may not necessarily be true, Gazzola said.

The primary somatosensory cortex is thought to represent how touch feels on the skin, but the findings suggest that its activity is modified by what the participant thought of the caresser, according to Ralph Adolphs, a professor of psychology and neuroscience at Caltech and director of the Caltech Brain Imaging Center, where the experiment was carried out.

"We see responses in a part of the brain thought to process only basic touch that were elicited entirely by the emotional significance of social touch prior to the touch itself, simply in anticipation of the caress that our participants would receive," said Adolphs.

These initial findings with only a small number of participants may not apply in a larger group. The researchers plan to test whether women's brains would respond the same way as men's did , and whether the brain would respond the same way across different sexual orientations.

"Nothing in our brain is truly objective," said Christian Keysers, a co-author of the study. "Our perception is deeply and pervasively shaped by how we feel about the things we perceive."